Method of obtaining low-phosphorus contents in medium-and high-carbon steels in a bottom-blown oxygen steelmaking furnace

ABSTRACT

A METHOD FOR DIRECTLY PRODUCING MEDIUM AND HIGH CARBON STEELS WITH LOW PHOSPHORUS CONTENTS IN A BOTTOM BLOWN BASIC STEEL REFINING PROCESS IN WHICH FINELY DIVIDED LIME IS ENTRAINED IN THE OXYGEN STREAM. THE IMPROVEMENT COMPRISES EMPLOYING AN OVERALL AVERAGE LIME LOAD RATE(LO) THROUGHOUT THE WHOLE BLOW OF ABOUT 0.09 TO 0.16LBS. LIME/FT.3 OF OXYGEN, WHEREIN THE AVERAGE RATE FOR THE FIRST HALF OF THE BLOW (L1) IS LESS THAN LO AND THE AVERAGE RATE FOR THE SECOND HALF OF THE BLOW (L2) IS GREATER THAN LO. PHOSPHORUS REDUCTION IS ADDITIONALLY ENHANCED BY DECREASING THE BLOWING TME, I.E., INCREASING THE OXYGEN BLOWING RATE.

Filed Nov. 13, L972 L/ME L040 RATE (lbs/I7 D. A. DUKELOW E 3,826,647 METHOD OF OBTAINING LOW-PHOSPHORUS CONTENTS IN MEDIUM AND HIGHCARBON STEELS IN A BOTTOM-BLOWN OXYGEN STEELMAKING FURNACE 5 Sheets-Sheet l rLo FRACT/O/V 0F BLOW/N6 PER/0D l I l l l l FRACTION 0F BLOW/N6 PER/0D Jfly 30, 197

METHOD OF OBTAINING LOW-PHOSPHORUS CONTENTS IN MEDIUM AND HIGH-CARBON STEELS IN A BOTTOW-BLOWN OXYGEN STEELMAKING FURNACE Filed Nov. 15, 1972 um- L040 RA r5 {/05 m LIME LOAD RA r5 (lbs/H 5 Sheets-Sheet 2 l l l l FRACTION 0F BLOW/N6 PER/00 r/Lo J l I l l l l l l I FRACTION 0F BLOW/N6 PER/00 D. A. DUKELOW ET .My 11% if 3,826,647

METHOD OF OBTAINING LOW-PHOSPHORUS CONTENTS IN MEDIUM AND HIGH-CARBON STEELS IN A BOTTOM-BLOWN OXYGEN STEELMAKING FURNACE 5 Sheets-Sheet 5 Filed Nov. 13, L972 F R146 7' lO/V OF EL OWl/VG PER/00 July 30. 1974 D. A. DUKELOW Er METHOD OF OBTAINING LOW-PHOSPHORUS CONTENTS IN MEDIUM AND HIGH-CARBON STEELS IN A BOTTOM-BLOWN OXYGEN STEELMAKING FURNACE Filed Nov. 13, 1972 LIME LOAD RA TE (/bs/f/ 5 Sheets-Sheet 5 FIG! .9

United States Patent 3,326,647 METHOD OF OBTAINING LOW-PHOSPHORUS CONTENTS 1N MEDIUM- AND HIGH-CARBON STEELS IN A BOTTOM-BLOWN OXYGEN STEELMAKING FURNACE Donald A. Dukelow, Mount Lebanon Township, Allegheny County, Phillip B. Hunter, Plum Borough, and Robert J. King, Churchill Borough, Pan, assiguors to United States Steel Corporation Continuation-impart of abandoned application Ser. No. 205,353, Dec. 6, 1971. This application Nov. 13, 1972, Ser. No. 306,138

Int. Cl. CZlc 7/02 US. Cl. 75-60 16 Claims ABSTRACT OF THE DISCLOSURE A method for directly producing medium and high carbon steels with low phosphorus contents in a bottom blown basic steel refining process in which finely divided lime is entrained in the oxygen stream. The improvement comprises employing an overall average lime load rate (L throughout the whole blow of about 0.09 to 0.16 lbs. lime/ft. of oxygen, wherein the average rate for the first half of the blow (L is less than L and the average rate for the second half of the blow (T is greater than L Phosphorus reduction is additionally enhanced by decreasing the blowing time, i.e., increasing the oxygen blowing rate.

This application is a continuation-in-part of Application Ser. No. 205,353, filed Dec. 6, 1971, and now abandoned.

A new basic-steelmaking process, the bottom blown process, is beginning to receive considerable attention in the United States. In a manner similar to the conventional BOP process, this new process utilizes a combination of an oxygen blow and a lime-containing basic slag to remove impurities from molten pig iron. Unlike the Basic Oxygen Process, this new process blow oxygen through tuyeres located below the surface of the molten metal. In one embodiment, the tuyeres are located in the bottom of the vessel, in a manner similar to that of the Bessemer process. In another embodiment, a similar tuyere system is employed in the side of the vessel or hearth. Combinations of these embodiments may also be employed. However, these various modifications are all based on the use of a protected tuyere system. In general, these new tuyeres are comprised of a generally concentric tube system in which the central tuyere is employed for the oxygen stream. Each oxygen tuyere is surrounded by a larger tuyere for the simultaneous injection of a protective coolant fluid which serves to jacket the oxygen stream. This jacket 'fluid may be natural gas, other hydrocarbon containing liquids or gases or in some instances relatively non-reactive type gases such as argon, ammonia or carbon monoxide. The jacket fluid acts as a coolant, reducing the rate of reaction between the molten metal and oxygen adjacent the tuyere and thereby preventing the rapid erosion of both the tuyere itself and the adjacent refractory. For purposes of this invention the term bottom blown process, will therefore include all processes in which at least one tuyere system is located between the surface of the molten steel. Generally, lime powder is entrained in the oxygen stream and blown into the bath to flux the oxidized impurities. In order to achieve low residual levels of undesirable impurities such as phosphorus and sulfur, it is conventional (in European practice) to provide an extensive oxygen blow and thereby decrease the carbon to a rather low residual level. Thus, if it is desirable to produce a. steel with a carbon level in excess of about 0.2%, it is then necessary to recarburize the sorefined steel. This practice is of course extremely wasteful, in that substantial amounts of oxygen are utilized unnecessarily in removing carbon. which must eventually be replaced. Furthermore, the correspondingly increased blowing period results in increased wear on both the tuyeres and furnace linings. Equally important, recarburizing the steel is somewhat hazardous in that a large amount of gas is emitted during the addition of carbon, often resulting in molten steel being blown from the ladle. In the Basic Oxygen Process, these problems have been overcome in many instances by employing a catch carbon practice, in which the steel is blown with an amount of oxygen only sufficient to achieve the desired final level of carbon (generally in excess of 0.35%). In the BOP process this practice has generally been successful in removing substantial amounts of both phosphorus and sulfur. This is believed to be due to the fact that the impurities in the upper portion of the melt are depleted more rapidly in this process, thereby creating a substantial, nonequilibrium condition in the melt. Because of this nonequilibrium condition, particularly during the period of heavy carbon oxidation, appreciable quantities of iron are oxidized and transferred to the slag. The resulting high iron oxide concentrations in the slag then prevent the reversion of phosphorus already present in the slag, back to the melt. Accordingly, low levels of phosphorus can be achieved in the BOP, without the necessity of eliminating substantially all the carbon.

On the other hand, the bottom blow processes achieve substantially enhanced mixing during the blow, hence equilibrium conditions are more closely approached. While this is beneficial in improving the yield (substantially less iron oxide in the slag), the amount of phosphorus in the bath at the end of the blow is considerably greater. Thus, in the bottom blown process, phosphorus has only been substantially removed (i.e. greater than about reduction) after the carbon has been reduced to rather low levels. In view thereof, in utilizing the bottom blow proc ess, the above-mentioned benefits of catch carbon" practice have not been available to the art in many instances. For example, in utilizing pig irons with phosphorus contents in excess of about 0.14%, the production of a steel with phosphorus below about 0.025% has not been readily achievable. Similarly, even with pig irons with low initial phosphorus contents (e.g. 0.04 to 0.06% it is often desirable to produce a steel with only about 0.005 to 0.01% phosphorus.

It is therefore a principal object of this invention to provide a method for achieving substantial phosphorus reduction in bottom blown steelmaking processes, while employing a catch carbon practice.

This and other objects and advantages of the invention will be readily understood from a reading of the following description when taken in conjunction with the appended claims and the drawings, in which FIGS. 1 through 9 are graphical representations of the lime load patterns employed in the examples of Table I.

It is generally accepted that phosphorus could be reduced substantially, if rather excessive amounts of lime were to be employed. See, for example, F. Shenouda et a1., Iron and Steel, June 1971, pp. 167-171. The addition of such excessive amounts of lime (i.e., greater than 0.2 lbs/ft. oxygen) would however be counterproductive. Obviously, the employment of excessive lime would increase costs of raw material usage. More importantly, excessive lime results in increased slag volume and a concommitant decrease in yield (more iron oxide in the slag). Similarly, the resultant slags are crumbly, further tending to decrease yield.

It has now been found that phosphorus reductions in excess of 80% are consistently achievable in catch carbon practice while employing average lime loads below 3 about 0.16 lbs. of lime per ft. of oxygen. This finding is based on the use of a particular lime load pattern in which the average lime load rate for the first half of the blowing period is significantly less than average for the second half of the blowing period. More specifically, if TABLE H the total average lime load for the whole of the blowing I T W F, 1P T tal h P ng Period is Within the range of about to Example number r ie r bent p i eent t i rii rzfi hc fid n lbs./ft. and preferably from about 0.11 to 0.14 lbs./ft.

substantial phosphorus (and sulfur) reductions are 332; 8:32? 3:2 achieved if the average for the first half of the period 0070 0024 13.0 66 (L is at least less than L 'and the average for the 81 818% 3:3 second half of the period (L is at least 10% greater g-gg 8' 81% 3-? than L Further enhancement of phosphorus removal is g1 g 1 1 3: 3

achieved if: (a) L, is at least less than L and L is at least 20% greater than L and/or (b) the oxygen blowing rate is sufficiently high to achieve the desired final level of carbon in a period of less than about 10 minutes, preferably less than eight minutes. In order to achieve the benefits of the above practice, it is also necessary that the end-point temperature of the metal bath be kept within rather specific limits. The desired end-point temperature is primarily dependent of the final, desired carbon level of the steel. Methods for controlling the end-point temperature, such as by proper control of charge (e.g. hot metal to scrap ratio) are well known to the art. Therefore consistent with good pouring practice, the end-point temperature (T should not exceed that given by the following equation:

max. F.)=303o-100 c where C is the desired content in percent carbon.

Even greater consistency of phosphorus removal will be achieved if the end-point temperature is below that given by:

T F.)=2940100(C) Thus, for example, if the desired level of carbon is 0.5%, then it is most preferable that the end-point temperature be below T ;=294010O (.5), or 2890 F.

Illustrative examples of a vtriety of lime load patterns are shown in Table 1, and in corresponding FIGS. 1 through 9. Examples 1 to 4 are representative of patterns outside the scope of this invention, while Examples 5 to 9 describe those practiced in accord with the teachings of this invention.

It may be seen from the above, that while the total average lime load for example 2 (see FIG. 2) was greater than for any of the inventive examples, the phosphorus reduction was clearly inferior. The exceedingly poor performance of this example is the result of both an undesirable lime load pattern, coupled with a comparatively long blowing period. This example should be compared with that of number 6, in which the pattern employed, although falling within the scope of the invention, was not within the more preferred range (i.e., L less than 20% of L for the first half and L greater than 20% of L for the second half of the period). Here, however, a phosphorus reduction in excess of 90% was nevertheless achieved due to the increased oxygen blowing rate, i.e. a total blowing period of only 5.8 minutes. Example 9 is illustrative of a further preferred embodiment of the invention, in which finely divided iron oxide was simultaneously injected together with the lime during final portion of the blow, preferably during the final 25% of the period. The patterns employed in Examples 5, 7 and 8 are illustrative of particularly preferred embodiments of the instant invention, wherein the lime load rate for about the last 40% of the blowing period is equal to or greater than about 1.5 times that of L A number of other modifications of the prescribed practice may also be employed. Thus, when the sulfur content of the pig iron is high, i.e., greater than 0.05%, it may be desirable to employ a comparative high lime load rate (i.e., greater than L during about the first 25% of the blow. Desulfurization is favored by reducing condi- TABLE I.HEAT DATA Steel Pig iron (liquid) Blowing Lime percent time, loading, Composition, percent min., Oxygen, lbs./ Temp,

Example number Charge materials, lbs. I S Si percent s.c.t'.m. s.e.f. Oz 0 P S Si F 1 58,300 liq. pig iron 0. 23 1 O. 048 0.79 8 (60) 3, 000 0. 11

4,600 Steel scrap 3. 7 (20) 3,000 0.04 0.85 0.138 0.016 0.016 2,830

1,000 solid pig iron 1. 5 (11) 3,000 0. 0 0.53 0. 063 0.019 0.008 2, 800

2 57,900 liq. pig iron 0. 065 0.031 0.93 5 8 (39) 3,000 0. 17

2,000 Steel Scrap 5 4 (37) 3, 000 0.08

1,200 solid pig iron 3 2 (21) 3,000 0.17 0. 69 0. 045 0.016 0 003 2,800

3 48,900 liq. pig iron. 0.070 0.031 1.12 5.0 (38) 3, 000 0. 0

10,000 steel scrap. 6. 2 (47) 3, 000 0.17

1,200 solid pig iron. 1.8 (15) 3,000 0.0 0.50 0. 024 0.024 0.003 2,880

4 53,200 lig. pig iron. 0. 070 0. 032 1. 10 6. 8 (50) 3,000 0. 04

7,900 steel scrap .3

1,200 solid pig iron 1 5 42,600 liq. pig iron .'5

7,100 steel scrap 5 1,200 solid pig iron-..

6 30,300 liq. pig iron.

8,300 steel scrap 1,200 solid pig iron.

7 38,100 liq. pig iron 11,500 steel scrap. 0. 18 0. 54 0. 013 0. 013 0. 003 2, 860

1,200 solid pig iron 8 38,10 liq. pig iron 0.180 0.013 1.20 4. 9 (59) 3, 500

11,000 steel scrap 3. 4 (41) 3, 500

1,200 solid pig iron 9 37,800 liq. pig iron.... 0.180 0. 02s 1. 34 2 (25) 3,500

11,000 steel scrap 5 (63) 3, 500

1,200 solid pig iron 1 (12) 3, 500

n Mixture of 50% finely divided iron oxide by weight injected with lime.

tions which are present when the silicon content of the metal bath is high, i.e., during about the first 25% of the period. If a high lime load is, in fact, employed during the first 25% of the blow, say 0.15 lbs./ft. it is nevertheless essential that the average for the first 50% of the blow be within the prescribed practice. Thus, a lime load of, for example, 0.04 lbs./ft. could be employed for the second 25% of the blowing period.

In the illustrative examples, the lime for any period was injected at a constant rate, i.e. the slope of the patterns shown in the figures is zero. However, the invention is not limited to the use of such zero slope load patterns. Thus, for any given portion of the blow, the lime may also be added at an increasing rate (positive slope) or at a decreasing rate (negative slope). It is only essential for purposes of this invention that the area under the curve for the whole blow, provide an average lime load rate L,

of 0.09 to 0.16 lbs./ft. and that for the first half of the blow the area yield an average rate at least less than L while the area for the second half of the blow yields an average value at least 10% greater than L We claim:

1. In the bottom blown process for the refining of molten iron, wherein carbon and other impurities are oxidized by the introduction of a stream of oxygen containing gas, said gas being introduced so as not to decrease the carbon in the resulting refined molten steel to a level below about 0.2% and wherein finely divided lime is entrained within said oxygen containing gas stream,

the improvement which comprises adding said lime at a rate to achieve an overall lime load rate L of 0.09 to 0.16 pounds of lime per ft. of oxygen, wherein the average lime load rate for the first half of the blowing period L is at least 10 percent less than L and the average lime load rate for the second half of the blowing period L is at least 10 percent greater than L and controlling the end-point temperature of said refined molten steel so that it does not exceed that given by the equation:

max. (F.)=3030 100 c) where C is the desired level of carbon in percent.

2. The method of Claim 1, wherein at least one of said composite tuyeres is located in the bottom of the steelmaking vessel.

3. The method of Claim 2, wherein L is within the range of 0.11 to 0.14 pounds of lime per ft. of oxygen.

4. The method of Claim 3, wherein said oxygen is blown at a rate sufficient to achieve the desired level of carbon in a period of less than about 10 minutes.

5. The method of Claim 2, wherein said desired carbon level is greater than 0.35 percent, and L is at least 6 20 percent less than L and L is at least 20 percent greater than L 6. The method of Claim 5, wherein said oxygen is blown at a rate sufiicient to achieve the desired level of carbon in a period of less than about 10 minutes.

7. The method of Claim 5, wherein said end-point temperature is less than that given by the equation:

max,

8. The method of Claim 5, wherein the average lime load rate for about the last 40 percent of the blowing period is equal to or greater than about 1.5 times L 9. The method of Claim 8, wherein said oxygen is blown at a rate sufficient to achieve the desired level of carbon in a period of less than about 10 minutes.

10. The method of Claim 8, wherein said end-point temperature is less than that given by the equation:

T (F.)=2940-100 c 11. The method of Claim 10, wherein said oxygen is blown at a rate sutficient to achieve the desired level of carbon in a period of less than about 10 minutes.

12. The method of Claim 5, wherein said iron to be refined is pig iron with a sulfur content in excess of about 0.05 period and the average lime load for about the first 25 percent of the blowing period is equal to or greater than L 13. The method of Claim 5, wherein said iron to be refined is pig iron with a phosphorus content in excess of about 0.14 percent.

14. The method of Claim 13, wherein said end-point temperature is less than that given by:

max. ("F.)=2940 -100 (c 15. The method of Claim 14, wherein the average lime load rate for about the last 40 percent of the blowing period is equal to or greater than 1.5 times L 16. The method of Claim 15, wherein said oxygen is blown at a rate sufficient to achieve the desired level of carbon in a period of less than about 10 minutes.

References Cited UNITED STATES PATENTS 3,330,645 7/1967 De Moustier 60 2,864,689 12/1958 Perrin 75-60 3,556,773 1/l971 Grenfell 7560 2,918,365 12/1959 Kanamori 75-60 L. DeWAYNE RUTLEDGE, Primary Examiner PETER D. ROSENBERG, Assistant Examiner US. Cl. X.R. 75-5 5 

